Tonometer tip and use of same

ABSTRACT

Ophthalmological device including an applanation tonometer tip having a bi-curved cornea-contacting surface and method of using such device for measurement of intraocular pressure. The cornea-contacting surface includes a first rotationally symmetric portion a curvature of which is substantially adapted to that of a typical cornea and a second rotationally symmetric portion that is peripheral to and adjoining the first portion. In operation, the applanation of the cornea in an area corresponding to the first portion of the cornea-contacting surface is substantially negligible.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation from the U.S. patentapplication Ser. No. 14/012,592 filed on Aug. 28, 2013 and published asUS 2014/0073897, which in turn claims priority and benefit from the U.S.Provisional Patent Application No. 61/699,062 filed on Sep. 10, 2012.The disclosure of each of the above identified patent applications isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an ophthalmological instrument and,more particularly, to a tip (for applanation tonometer that isstructured as a cornea-contacting member) and the applanation tonometerutilizing such tip.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide an ophthalmologicalinstrument comprising a corneal contact member having a front surfacethat is adapted, in operation, to face the cornea of an eye. The frontsurface of the corneal contact member includes (i) a corneal contactsurface portion having a concave shape, which corneal contact surfaceportion defines a central portion of the front surface of the cornealcontact member; and (ii) a peripheral surface portion surrounding thecorneal contact surface portion and tangentially merging with thecorneal contact surface portion along a closed plane curve. In oneembodiment, curvatures of the peripheral surface portion and the cornealcontact surface portion have opposite signs. The concave shape of thecorneal contact surface may be approximately adapted to the shape of thecornea such as to establish, in operation, an areal contact (i.e., acontact characterized by an area) between the front surface and thecornea within a bound of the closed plane curve. In a specificembodiment, the closed plane curve defines a circle. The front surfaceis generally configured to applanate a portion of the cornea when thecorneal contact member is pressed, in operation, against the cornea,such that the applanated portion of the cornea defines an annulus aroundthe closed plane curve.

In one embodiment, the corneal contact member is adapted to illuminateand image the cornea of the eye in light passing through the cornealcontact member and, in a specific implementation, the corneal contactmember is adapted to from a double image of the cornea in lightreflected by the cornea. The corneal contact member may be configured asa tip of a Goldmann-type tonometer, with the front surface including asurface that is rotationally symmetric about an axis and having across-section that defines a curve having first and second radii ofcurvature of opposite signs.

In a related implementation, the front surface is devoid of openingstherethrough and the peripheral surface portion deviates from a plane.Embodiments of the invention also provide a tonometer including theabove-described ophthalmological instrument.

Embodiments of the invention further provide an ophthalmologicalinstrument comprising a corneal contact member having a front surfacethat is adapted, in operation, to face the cornea of an eye, and that isrotationally symmetric about an axis. Such corneal contact membergenerally includes (i) a corneal contact surface portion defining aportion of spherical surface that is devoid of openings therethrough,such that the corneal contact surface portion is approximately adaptedto the shape of the cornea; and (ii) a peripheral surface portionsurrounding the corneal contact surface portion and tangentially mergingwith the corneal contact surface portion along a closed curve defined ina plane that is perpendicular to the axis. The ophthalmologicalinstrument may be adapted (a) to illuminate and image the cornea inlight transmitted through the corneal contact member when this member isbrought in contact with the cornea along the corneal contact surfaceportion. In addition or alternatively, such ophthalmological instrumentmay be adapted to applanate an annular portion of the cornea when thecorneal contact member is pressed against the cornea such as to formcontact with the cornea along the corneal contact surface portion, andsuch as to define the annular portion of the cornea around theabove-defined closed curve. In one implementation, the ophthalmologicalinstrument is adapted to maintain a curvature of a portion of thecornea, which is in contact with the corneal contact surface portion,substantially unchanged when the corneal contact member is pressedagainst the cornea.

Embodiments of the invention further provide a method for measuringintraocular pressure with an applanating tonometer that includes acorneal contact member (which corneal contact member has a cornealcontact surface with a shape substantially adapted to a shape of thecornea of an eye). The method includes the steps of (i) pressing thecorneal contact surface against the cornea to establish a contactbetween the corneal contact surface and a first portion of the corneaand to applanate a second portion of the cornea while keeping thecurvature of the first portion of the cornea substantially unchanged,such that the second portion defines an annulus around the firstportion; and (ii) imaging the cornea in light traversing the cornealcontact member to form an image. The method further includes determininga value of the intraocular pressure based on imaging data representingso formed image. In one embodiment, the step of pressing includespressing a rotationally symmetric surface of the corneal contact memberagainst the cornea. Alternatively or in addition, the step of pressingincludes pressing a rotationally symmetric surface having first andsecond radii of curvature of opposite signs. Alternatively or inaddition, the step of pressing includes pressing a surface, of thecorneal contact member, that is rotationally symmetric about an axisalong which light traverses the corneal contact member to effectuateimaging of cornea. Alternatively or in addition, the step of pressingincludes pressing a surface devoid of openings therethrough. The step ofimaging may include imaging through a bi-prismatic optical element. Inone implementation, the step of determining does not include correctionof an offset value caused by a corneal characteristic related to one ormore of corneal thickness and corneal stiffness. Generally, anembodiment of the method is devoid of using a pachymeter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by referring to thefollowing Detailed Description of Specific Embodiments in conjunctionwith the Drawings, of which:

FIG. 1A presents two views of Goldmann applanation tonometer top showingthe bi-prism angle for the human (60 degrees);

FIG. 1B is a diagram illustrating a Goldmann applanation tonometer;

FIG. 2A is a diagram illustrating flattening of the corneal surface dueto pressure applied by the tonometer tip;

FIG. 2B is a diagram showing the pressure-dependent positioning of twosemi-circles representing an image of the flattened portion of thecorneal surface;

FIGS. 3A and 3B are views that illustrate schematically a tonometer tipaccording to an embodiment of the invention;

FIG. 3C is a diagram illustrating an alternative embodiment of theinvention;

FIG. 4 is a diagram illustrating a method for measurement of intraocularpressure with an embodiment of FIGS. 3A, 3B.

DETAILED DESCRIPTION

The problem of performing the measurement of intraocular pressure withthe use of a Goldmann-type tonometer without the need to correct for thecontribution of corneal thickness and stiffness is solved with the use atonometer tip the cornea-contacting surface of which includes a centralportion having a concave shape and a peripheral portion encircling thecentral portion and having a curvature with a sign opposite to the signof the curvature of the central portion, the central and peripheralportions merging tangentially along a closed plane curve. In accordancewith embodiments of the present invention, methods and apparatus aredisclosed for an ophthalmological instrument including a corneal contactmember for use with the Goldmann tonometer platform. Embodiments of theinvention include a tonometer tip that includes a conventionally usedbiprism-containing portion and a corneal contact surface that isconfigured to minimize deformation of the corneal surface during thecornea applanation procedure.

Tonometry is a non-invasive procedure that eye-care professionalsperform to determine the intraocular pressure (IOP), the fluid pressureinside the eye. It is an important test in the evaluation of patients atrisk from glaucoma, which may cause visual impairment in a patient. Inapplanation tonometry the intraocular pressure is inferred from theforce required to flatten (applanate) a constant, pre-defined area ofthe cornea, as per the Imbert-Fick hypothesis that holds that when aflat surface is pressed against a closed sphere with a given internalpressure, an equilibrium will be attained when the force exerted againstthe spherical surface is balanced by the internal pressure of the sphereexerted over the area of contact. In other words, pressure P within aflexible, elastic (and presumably infinitely thin) sphere isapproximately equal to the external force f required to flatten aportion of the sphere, normalized by an area A that is flattened, P=f/A.Accordingly, a transparent pressure member with a planar contact surface(such as the element 100 as shown in FIG. 1A, for example) is pressedagainst the cornea of an eye in such a way that the latter is flattenedover a pre-determined area that in practice should be about 7.3 mm².

Before performing the measurement, and because the pressure member makescontact with the cornea, a topical anesthetic (such as proxymetacaine)is typically introduced on to the surface of the eye (for instance, inthe form of eye drops). During the measurement, the eye is illuminatedby blue light (delivered from a lamp with a blue filter in front of it,for example). In the zone of contact between the surface of the corneaand the pressure member, the film of tears (which contains fluoresceinand has green-yellowish hue when illuminated with the blue light) isdisplaced, as a result of the contact, so that the boundary between theflattened and the curved areas of the cornea is readily identifiable.The contact pressure required for flattening is used as a measure ofintraocular pressure.

The classical Goldmann tonometer (see an example 114 in FIG. 1B) has atransparent plastic applanating tip 100 shaped as a truncated cone,through which the surface of cornea 120 is observed with the slit-lampmicroscope, and is the most widely used version in current practice oftonometry that utilizes the applanation of the cornea 120. The pressuremember, or corneal contact member, also referred to as a tip 100typically contains a bi-prism (two prisms touching at their apices),which, in reference of FIG. 2A, produces optical doubling of the imageof the flattened surface 202 and separates the two image components by afixed distance or space, across the field of view, that is dependent onthe apex angles of the prisms. In further reference to FIG. 1B, theGoldmann tonometer corneal contact member or tip 100 is connected by alever arm to the tonometer body 116. The tonometer body 116 contains aweight that can be varied.

The observer-examiner uses an optical filter (usually, a cobalt bluefilter) to view the two image components (shown as semicircles 210A,210B in FIG. 2B). The force applied through the tonometer tip 100 to thesurface 220 of the cornea 120 is then adjusted using a dial (knob)connected to a variable tension spring of the device until the inneredges of the semicircles 210A, 210B viewed in the viewfinder are made tomeet or coincide (see insert of FIG. 2B). Such meeting of the edgesoccurs when a corneal area of about 3.06 mm in diameter has beenflattened and when the two opposing, counteracting forces (the firstproduced by resistance of the rigid cornea and the second by the tensionof the tear film) become substantially equal and cancel each other out,thereby allowing the pressure in the eye to be determined from the forceapplied.

Like all non-invasive methods, this method of determining an intraocularpressure is inherently imprecise. Some of the measurement errors arisedue to the fact that a cornea, unlike the ideal sphere, has non-zerothickness: a thinner than average cornea typically results in anunderestimation of the IOP, while a thicker than average cornea mayresult in an overestimate of the actual IOP. To counterbalance thenon-zero stiffness of the cornea and in order to applanate a portion ofthe cornea, additional force is required that cannot be counted towardsthe actual value of IOP. The studies revealed a correlation between thecorneal thickness and corneal stiffness. Accordingly, to reduce theIOP-measurement error, the value of the initially measured applied forcehas to be corrected in reference to a second measurement of cornealthickness performed using a pachymeter. The accuracy of such correctionis predicated upon the accuracy of correlation between the thickness andstiffness characteristics of the cornea, which is also inherentlyinaccurate (due to influence of such variable factors as age of theperson, a diameter of the cornea, corneal curvature, and effectsproduced by various eye diseases).

Therefore, a measurement technique and system that increase theprecision and accuracy of the IOP results are required. The use ofembodiments of the present invention increases the accuracy of themeasurement of the IOP with the use of a Goldmann applanation tonometer,thereby eliminating a need in an auxiliary measurement of the cornealthickness and reducing the overall cost of the IOP measurement andincreasing the quality of care.

In reference to FIGS. 3A and 3B, a relevant portion 300, representing atip of an embodiment of an optical element designed to be brought incontact with the cornea of an eye (and referred to as corneal contactmember), is shown in a partial cross-sectional view and a front view,respectively. A corneal contact surface 304 includes a central concavesurface portion 304A, which is optionally adapted to and is congruentwith the curvature of the cornea and which in a specific embodiment mayinclude a surface that is rotationally symmetric about an axis 306. Theexample of FIGS. 3A and 3B shows just such rotationally symmetric case.In operation, the central concave surface portion 304A is brought incontact with the corneal surface 220. At a periphery of the cornealcontact surface 304, the central concave surface portion 304A passesover into a peripheral surface portion 304B that has a curvature of anopposite sign (as compared to that of the central surface portion 304A).The peripheral surface portion 304B defines a looped (and in thespecific case—annular) projection, along the axis 306 and onto a planetransverse to the axis 306. The central concave surface portion 304A andthe peripheral annular portion 304B are tangentially, seamlessly merginginto each other along a closed curve 310 defined in a plane that istangential to the surface 304 and perpendicular to the axis 306. Putdifferently, a first plane, which is tangential to the central surfaceportion 304A at the boundary 310 between the surface portions 304A, 304Band a second plane, which is tangential to the peripheral surfaceportion 304B at the boundary 310 that is shared by the surface portions304A, 304B, substantially coincide with one another and do not form adihedral angle.

In one implementation, and in further reference to FIGS. 3A, 3B, theconcave surface portion 304A includes a spherical surface having aradius of curvature R of about −9.0 mm (defined in a plane containingthe axis 306), and a footprint or normal projection along the axis 306with a diameter d of about 3.06 mm (defined in a plane perpendicular tothe axis 310). The peripheral annular (i.e., having a form of a ring)surface portion 304B has a radius of curvature of about 3.0 mm (definedin a plane containing the axis 306). In such implementation, thefootprint or projection of the corneal contact surface 304 onto theplane normal to the axis 306 defines a circle with a diameter D of about6.0 mm. The corneal contact surface 304 may include a polymeric surface(for example, polycarbonate, with a refractive index on the order of1.5) or a glass surface with polished finish of substantially opticalquality. Generally, a lateral boundary or perimeter 320 of the surface304 does not need to meet any particular optical, mechanical, orgeometrical requirement.

In a related embodiment, the corneal contact surface is modified such asto have different extents in different directions and, generally, anon-axially-symmetric footprint or normal projection. The centralconcave surface portion of the corneal contact surface, while remainingsubstantially fitted (curvature wise) to the corneal surface, may haveunequal extents in two (in a specific case—mutually perpendicular)directions. Accordingly, the peripheral surface portion, while remainingadjoining to the central concave surface portion in a fashion describedabove, also has a ratio of lateral extents that is similar to the ratiocharacterizing the central concave portion.

In a specific example shown in FIG. 3C, the so-configured cornealcontact surface 350 has an elliptical or oval footprint 352 on a planethat is perpendicular to the z-axis. The surface 350 includes a central,substantially spherical surface portion 354A and a peripheral annularportion 354B, each of which has an elliptically-shaped correspondingprojection on the plane that is perpendicular to the axis 306 (which, inFIG. 3C, is parallel to the axis z of the indicated local system ofcoordinates). As shown, the dimensions of the central surface portion354A along the minor and major axes of the corresponding footprint are aand b, respectively. The maximum dimensions of the peripheral surfaceportion 354B along the corresponding minor and major axes of itsfootprint are A and B, respectively, and indicated by a perimeter 320′.The surface portions 354A, 354B are tangentially, seamlessly merginginto one another along an elliptical close plane curve 310′ in a fashionsimilar to that described in reference to FIGS. 3A and 3B. In oneimplementation, a is about 2.13 mm, b is about 3.06 mm. The bi-prismaticelement (not shown) that is internal to the corneal contact memberhaving the surface 350 may be oriented such as to approximately bisectthe long extent B of the footprint 352 of FIG. 3C.

The implementation illustrated in FIG. 3C is adapted to facilitate themeasurements of the IOP of the patients with interpalpebral featuresthat may not necessarily allow the observer-examiner to accommodate asymmetrically-structured corneal contact surface of the embodiment ofFIGS. 3A and 3B. It is appreciated that, when the implementation of theinvention the operation of which is represented by FIG. 3C is used inpractice, the area of the cornea subject to applanation remainssubstantially the same as that corresponding to the embodiment of FIG.3B. The lateral dimension of the oval footprint corresponding to 354Athat accommodates a narrow interpalpebral fissue (partially closed lids)is reduced, while the orthogonal dimension of the footprint (along theeye lids) is increased, as compared to the diameter of the footprint304A. Under some conditions, the force required to achieve applanationmay be reduced.

Referring now to FIG. 4, which show schematically the process of theexamination of an eye 400, during which a corneal contact member 300 isbrought in contact with the corneal surface 220. The corneal contactsurface 304, of the member 300, is shaped according to an embodiment ofthe invention and dimensioned to minimize the deformation of the cornealsurface 220 during the applanation procedure with the use of a Goldmanntonometer. In particular, and as will be understood by a skilledartisan, the minimization of the corneal deformation translates tominimization of the contribution of the corneal stiffness into the forcedefined by the eye in response to the applied measurement of the force(that, in turn, is required for proper applanation of a portion of thecorneal surface that defines a circular area with a diameter of about3.06 mm). As a practical result of such reduction or minimization of thecorneal contribution, the correction factor (which takes into accountcorneal thickness and that is used to practically unreliable compensatefor the unknown corneal stiffness, as discussed above) becomessubstantially negligible. The computational compensation of the errorsof the measurement of the IPO, therefore, becomes practicallyunnecessary. Similarly, a need to perform costly and time-consumingpachymetries, directed to correcting a cornea-thickness-related errorthat accompanies conventionally performed measurements of the IPO withthe use of the Goldmann tonometer, is substantially eliminated, therebyleading to a measurement method that does not include pachymetry.

Generally, a corneal contact surface of the corneal contact member 300is structured to include an azimuthally symmetric bi-curved surfacehaving a cross-section (in a plane containing an optical axis of thecontact member 300) defined by an axially-symmetric monotonic curvehaving first and second local maxima, one minimum that coincides withthe axis of symmetry of such curve, and a second derivative defined atany point of the curve. Such corneal contact surface includes a centralconcave portion and a peripheral convex portion that circumscribes thecentral concave portion. In operation, the central concave portion ofthe corneal contact surface produces a substantially negligiblecompression of the central portion of the cornea with which it comes incontact. A region of the corneal contact surface along which theperipheral convex portion and the central contact portion adjoin eachother produces a slight corneal compression to define a peripheral ringpattern, observed in form of semicircles, in reflection of light fromthe cornea.

In further reference to FIG. 4, no particular components of theGoldmann-type tonometer are additionally shown for the simplicity ofillustration. The path of light, traversing the bi-prism-containingcorneal contact member 300 on its propagation from a light source 420,to a reflecting element 424, to the surface 220 of the cornea (and, inreflection, to an observer 430) is designated with arrows 440. Avariable pressure force, applied to the corneal surface 220 isdesignated with an arrow 450.

It is understood that specific numerical values, chosen for illustrationof examples of embodiments described in reference to FIGS. 3A, 3B, and4, may generally vary over wide ranges to suit different applications.It will be understood by those of ordinary skill in the art thatmodifications to, and variations of, the illustrated embodiments may bemade without departing from the inventive concepts disclosed herein.Both of the central concave surface portion and the associatedperipheral surface portion of the corneal contact surface may beuninterrupted and spatially continuous (such as the portions 304A, 304Bof FIGS. 3A, 3B or the portions 354A, 354B of FIG. 3C, for example).Alternatively, at least one of the central concave portion and theassociate peripheral surface portion may be spatially discontinuous (atleast in one direction transverse to the optical axis of the cornealcontact member) such as to define, in a projection onto a planeperpendicular to the optical axis of the corneal contact member, asegmented footprint of the corneal contact surface. For example, atleast one of the central concave surface portion and the peripheralsurface portion may be spatially interrupted such as to preservesymmetry of such interrupted surface portion(s) with respect to at leastone spatial axis. In reference to FIGS. 3A, 3B, and as a specificexample, the peripheral surface portion 304B may be spatiallyinterrupted along the y-axis. In operation, when pressed against thecornea, such segmented structure will define a plurality of applanationareas that are located substantially symmetrically about an axis alongwhich the surface interruption is present (in this case, along they-axis).

References throughout this specification to “one embodiment,” “anembodiment,” “a related embodiment,” or similar language mean that aparticular feature, structure, or characteristic described in connectionwith the referred to “embodiment” is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment. It is to be understood that no portion of disclosure, takenon its own and in possible connection with a figure, is intended toprovide a complete description of all features of the invention.

The use of terms “substantially”, “approximately”, “about” and similarterms in reference to a descriptor of a value, element, or property athand is intended to emphasize that the value, element, or propertyreferred to, while not necessarily being exactly as stated, wouldnevertheless be considered, for practical purposes, as stated by aperson of skill in the art. As understood by a skilled artisan, thepractical deviation of the exact value or characteristic of such value,element, or property from that stated may vary within a range defined byan experimental measurement error that is typical when using ameasurement method accepted in the art for such purposes.

In addition, the following disclosure may describe features of theinvention with reference to corresponding drawings, in which likenumbers represent the same or similar elements wherever possible. In thedrawings, the depicted structural elements are generally not to scale,and certain components are enlarged relative to the other components forpurposes of emphasis and understanding. It is to be understood that nosingle drawing is intended to support a complete description of allfeatures of the invention. In other words, a given drawing is generallydescriptive of only some, and generally not all, features of theinvention. A given drawing and an associated portion of the disclosurecontaining a description referencing such drawing do not, generally,contain all elements of a particular view or all features that can bepresented is this view, for purposes of simplifying the given drawingand discussion, and to direct the discussion to particular elements thatare featured in this drawing. A skilled artisan will recognize that theinvention may possibly be practiced without one or more of the specificfeatures, elements, components, structures, details, or characteristics,or with the use of other methods, components, materials, and so forth.Therefore, although a particular detail of an embodiment of theinvention may not be necessarily shown in each and every drawingdescribing such embodiment, the presence of this detail in the drawingmay be implied unless the context of the description requires otherwise.In other instances, well known structures, details, materials, oroperations may be not shown in a given drawing or described in detail toavoid obscuring aspects of an embodiment of the invention that are beingdiscussed. Furthermore, the described single features, structures, orcharacteristics of the invention may be combined in any suitable mannerin one or more further embodiments.

The invention as recited in claims appended to this disclosure isintended to be assessed in light of the disclosure as a whole, includingfeatures disclosed in prior art to which reference is made. Accordingly,the invention should not be viewed as being limited to the disclosedembodiment(s).

What is claimed is:
 1. A tonometer tip configured for use with atonometer, the tonometer tip comprising: a body having an axis and afront surface that is transverse to the axis and that is dimensioned tocontact a cornea of an eye, said front surface including a cornealcontact surface having a surface curvature with a first sign, saidcorneal contact surface configured to deform a first portion of thecornea to produce a counterforce applied by the cornea to the body whenthe corneal contact surface is pressed against the eye along the axis;and a peripheral surface having a surface curvature with a second sign,said second sign being opposite to the first sign, wherein said cornealcontact surface is centered on the axis, wherein said peripheral surfaceis co-axial with the corneal contact surface and surrounds the cornealcontact surface.
 2. The tonometer tip according to claim 1, wherein atleast one of the following conditions is satisfied: (i) the closed planecurve is defined in a plane that is transverse to the axis; and (ii)said tonometer tip includes an optical prismatic element.
 3. Thetonometer tip according to claim 2, wherein the optical prismaticelement includes two prisms, a first prism of the two prisms having afirst apex, a second prism of the two prisms having a second apex, thefirst and second apices touching each other.
 4. The tonometer tipaccording to claim 1, wherein at least one of the following conditionsis satisfied: a) the first sign is equal to a sign of a curvature of asurface of the cornea; and b) the front surface is axially symmetricabout said axis.
 5. The tonometer tip according to claim 1, wherein atleast one of the following conditions is satisfied: i) the cornealcontact surface defines a portion of a spherical surface; and ii) saidcorneal contact surface is devoid of openings therethrough.
 6. Thetonometer tip according to claim 1 configured as a tonometer tip of aGoldmann applanation tonometer.
 7. The tonometer tip according to claim1, wherein said front surface is dimensioned to maintain a curvature ofa second portion of the cornea substantially unchanged when the cornealcontact surface is pressed against the cornea.
 8. The tonometer tipaccording to claim 1, wherein at least one of the following conditionsis satisfied: a) said front surface is an azimuthally symmetricbi-curved surface having a cross-section, in a plane containing theaxis, that is defined by an axially-monotonic curve with a secondderivative defined at any point of said curve; and b) said peripheralsurface and said corneal contact surface are tangentially merged along aclosed plane curve.
 9. A method for using a tonometer tip with atonometer, the tonometer tip comprising a body elongated along an axis,the body having a front surface that is transverse to the axis and thatis dimensioned to contact a cornea of an eye, wherein the front surfaceincludes a corneal contact surface having a surface curvature with afirst sign, and a peripheral surface having a surface curvature with asecond sign, said second sign being opposite to the first sign, whereinsaid peripheral surface is co-axial with the corneal contact surface,encircles the corneal contact surface, and tangentially merges with saidcorneal contact surface along a closed plane curve, the methodcomprising: bringing the body against the cornea along the axis toestablish contact between the corneal contact surface and the cornea;deforming a first portion of the cornea to produce a counterforceapplied by the cornea to the body when the corneal contact surface ispressed against the eye along the axis; and imaging the eye in lighttraversing said tonometer tip and the corneal contact member to form animage containing imaging data.
 10. The method according to claim 9,comprising determining a value of intraocular pressure from said imagingdata.
 11. The method according to claim 10, the method being devoid of astep of correcting the imaging data to compensate for at least one of acorneal thickness and a corneal stiffness.
 12. A tonometer tipconfigured for use with a tonometer and through which pressure isapplied to a cornea of an eye during a measurement of intraocularpressure of the eye, the tonometer tip comprising: a front surfaceconfigured to contact the cornea during said measurement, wherein thefront surface is configured to maintain substantially unchanged acurvature of a central portion of the cornea in which an apex of thecornea is located, and wherein the front surface is further configuredto applanate a peripheral portion of the cornea outside of the centralportion of the cornea.
 13. The tonometer tip according to claim 12,wherein the peripheral portion of the cornea is an annular portion ofthe cornea.
 14. The tonometer tip according to claim 13, wherein theannular portion of the cornea is circular in shape.
 15. The tonometertip according to claim 14, wherein the annular portion of the cornea hasan inner diameter of about 3.06 mm in a plane normal to a measurementaxis along which pressure is applied to the cornea.
 16. The tonometertip according to claim 13, wherein the annular portion of the cornea iselliptical or oval in shape.
 17. The tonometer tip according to claim12, wherein the peripheral portion of the cornea includes a plurality ofareas.
 18. A tonometer tip for use with a tonometer and through whichpressure is applied to a cornea of an eye during a measurement ofintraocular pressure of the eye, the tonometer tip comprising a frontsurface for contacting the eye during measurement of intraocularpressure to applanate a portion of the cornea, wherein the front surfaceis configured to applanate a peripheral portion of the cornea withoutapplanating a central portion of the cornea in which an apex of thecornea is located.
 19. The tonometer tip according to claim 18, whereinthe peripheral portion of the cornea is an annular portion of thecornea.
 20. The tonometer tip according to claim 19, wherein the annularportion of the cornea is circular in shape.
 21. The tonometer tipaccording to claim 20, wherein the annular portion of the cornea has aninner diameter of about 3.06 mm in a plane normal to a measurement axisalong which pressure is applied to the cornea.
 22. The tonometer tipaccording to claim 19, wherein the annular portion of the cornea iselliptical or oval in shape.
 23. The tonometer tip according to claim18, wherein the peripheral portion of the cornea includes a plurality ofareas.
 24. An applanation tonometer for measuring intraocular pressureof an eye, the applanation tonometer comprising a corneal contact memberthrough which pressure is applied to a cornea of the eye along ameasurement axis during measurement of intraocular pressure, the cornealcontact member including a front surface for contacting the eye duringmeasurement of intraocular pressure to applanate a portion of thecornea, wherein the front surface is configured to maintainsubstantially unchanged a curvature of a central portion of the corneain which an apex of the cornea is located, and the front surface isfurther configured to applanate a peripheral portion of the corneaoutside of the central portion of the cornea.
 25. An applanationtonometer for measuring intraocular pressure of an eye, the applanationtonometer comprising a corneal contact member through which pressure isapplied to a cornea of the eye along a measurement axis duringmeasurement of intraocular pressure, the corneal contact memberincluding a front surface for contacting the eye during measurement ofintraocular pressure to applanate a portion of the cornea, wherein thefront surface is configured to applanate a peripheral portion of thecornea without applanating a central portion of the cornea in which anapex of the cornea is located.